®46 WONDERS or ART 
which is disengage'd during the working of the coals, an^ 
■from fissures in the strata ; and which, when it has accurnU' 
lated so as to form more than l-13th part of the volume of 
the atmospherical air, becomes explosive by a lighted caudle> 
or by any kind of flame. The apertures in the gauze should 
not be more than 1 -20th of an inch square. As the firC' 
damp is not inflamed by ignited wire, the thickness of the 
wire is not of importance, but wire from l-40ih to l-60lh 
of an inch in diameter is the most convenient. If the wire 
of l-40th is found to wear out too soon in practice, the 
thickness may be increased to any extent ; but the thicket 
the wire, the more the light will be intercepted, for the s\f-o 
of the apertures must never be more than I -20th of an inch 
square. In the svorking models which he has sent to the 
mines, there are y48 apertures in the square inch. 
W^hen the wire-gauzc-safe-lamp is lighted and introduced 
into an atmosphere gradually mixed with fire-damp, the fii's^ 
effect of the fire-damp is to increase the length and size of 
the flame. When the inflammable gas forms as much 
l-12th if the volume of the air, tiro cylinder becomes filled 
with a feeble blue flame, but the flame of the wick appeat* 
burning brightly within the blue flame, and tire light of >1'® 
wick continues till the fire-damp increases to l-6th or •' 
5th, when it is lost in the flame of the fire-damp, which >t* 
this case fills the cylinder with a pretty strong light. 
long as any explosive mixture of gas exists in contact with 
the lamp, so long it will give light, and when it is exOn' 
guib'hed, which liappens when tlie foul air constitutes a* 
much as l-3d of the volume of the atmosphere, the air is 
longer proper for respiration. In cases in which the 
damp is mixed only in its smallest explosive proportion with 
air, the use of the wire-gauze-safe-lamp, which rapidly coh* 
suraes the inflammable gas, will soon reduce the quanh'y 
below the explosive point ; and it can scarcely ever happfhi 
that a lamp will be exposed to an explosive mixture contaih' 
ing the largest proportion of fire-damp : but even in this cass 
tire insti'ument is absolutely safe; and should the 
become red-hot, they have no power of communicatii'S 
explosion. Should it ever be necessary for the miner h) 
work for a great length of time in an explosive atmosph^''® 
by the wire-gauze-safe-lamp, it may be proper to cool t'j® 
lamp occasionally by throwing water upon the top, or a 
